[meta] Migration to media

Problem/Motivation

As a follow up of #2831274: Bring Media entity module to core as Media module and as part of #2825215: Media initiative: Essentials - first round of improvements in core we will need to provide migrate integration for the media entity.

This is a meta issue destined to track progress on all sub-tasks that this plan entails.

Proposed resolution

• Add a file source plugin to be able to source from all permanent files on the system
• Add media entity destination plugin to be able to migrate stuff into media entities.
• Provide example migration to migrate files into media entities. This would use the existing file entities, but then wrap them into media entities. (Probably in #2835825: Add d8 media migration too?)
• Provide migrations/update paths for file/image field configurations. They should become entity reference fields that reference the migrated media entities. #2885002: Provide an optional migration path for sites using files/images to media entities
• Provide a migration example for an actual file field
• Provide a migration example for an actual image field
• Provide a migration example for the file/image field display configuration.
• ... more?

Remaining tasks

User interface changes

API changes

Data model changes